In recent years, there has been an increasing demand for cost reductions in pipelines. As manufacturing techniques have progressed, high-strength steel pipes have come to be used in pipelines. The majority of high-strength steel pipes (straight pipes) used in pipelines have a strength up to API X70 grade. Only a few high-strength steel pipes of API X80 grade have been put to use worldwide. In particular, hot bend pipes which are manufactured by carrying out bending of a welded steel pipe having a weld extending in its axial direction are indispensable in the construction of actual pipelines. However, it is extremely difficult to guarantee both strength and toughness in welds by heat treatment performed after bending. Therefore, manufacturing techniques for high-strength hot bend pipes of at least API X70 grade have not been established.
Patent Document 1 discloses an invention for manufacturing a thick-walled high-strength hot bend pipe having a low weld cracking parameter Pcm by heating a thick-walled welded steel pipe having a specific composition such that the weld cracking parameter Pcm is at most 0.19% (in this description, unless otherwise specified, percent means mass percent) to a temperature of at least the Ac3 point to at most 1100° C., performing bending of the pipe at this temperature, then cooling the pipe at a specific cooling rate to 300° C. or below, and then tempering the pipe in a temperature range of at least 550° C. to at most 650° C.
Patent Document 2 discloses an invention for manufacturing a high-strength hot bend pipe having a weld metal with excellent toughness by heating a welded steel pipe having a weld metal in which the value of [{1.5(O−0.89Al)+3.4N}−Ti] (wherein O, Al, N, and Ti are respectively the contents of oxygen, aluminum, nitrogen, and titanium) is in the range of at least −0.010 to at most 0.010 to a temperature of at least 900° C. to at most 1000° C. immediately followed by rapid cooling while performing bending.
Patent Document 1: JP H05-67699 B
Patent Document 2: JP H09-295067 A